Japanese Pat. No. 4,733,082 discloses catalysts containing the oxides of molybdenum, phosphorus, and at least one element selected from the group consisting of arsenic, boron, silicon, cadmium, lead, tungsten, thallium, indium, germanium, and tin for the oxidation of unsaturated aldehydes to unsaturated acids, and preferredly for the oxidation of methacrylaldehyde to methacrylic acid. U.S. Pat. No. 3,875,220 discloses desirable catalysts containing phosphorus, vanadium, and molybdenum for the oxidation of methacrolein with molecular oxygen to obtain methacrylic acid. These catalysts may be optionally promoted with bismuth, arsenic, boron, cerium, chromium, silver, iron, tungsten, lead, manganese, thallium, tellerium, nickel, niobium, boron, tin and/or copper. German Pat. No. 2,048,602 discloses catalysts containing molybdenum, phosphorus, and elements such as tungsten, arsenic indium, and cadmium. The present invention is a result of a search for more efficient and desirable catalysts for the production of acrylic acid and methacrylic acid. Unexpected higher yields and selectivities of acrylic acid and methacrylic acid are obtained by the vapor phase oxidation of acrolein and methacrolein, respectively, with molecular oxygen in the presence of the new and useful catalysts of the present invention.